The present invention relates to a process for fabricating a magnetic recording medium and, more particularly, to a magnetic recording medium fabricating process suitable for use in fabricating a disk-type medium such as a floppy disk.
Generally, a magnetic recording medium such as a magnetic disk or tape is fabricated by applying magnetic liquid containing ferromagnetic fine particles dispersed in a binder dissolved in a solvent to a band-shaped nonmagnetic support continuously run in the longitudinal direction of the support subsequently drying and solidifying the magnetic liquid, and then punching or cutting the nonmagnetic support to the desired shape.
If, however, ferromagnetic fine particles are arrayed in a specific direction during fabrication of the magnetic disk so that the magnetic recording medium is magnetically anisotropic, the magnetic and electric characteristics in various directions on the disk will also be anisotropic. For example, let it be assumed that the magnetic particles are arrayed in the magnetic liquid application direction, that is, in the delivery direction of the nommagnetic support. In use, if the magnetic head scans not linearly but circumferentially in a concentric pattern, the signal level of the reproduced output in the application direction is higher than in other directions. As a result, the signal level of the reproduced output from the magnetic disk will vary as the disk rotates. (This is generally termed the "modulation" phenomena). Especially in case where the thickness of the magnetic layer varies, this effect is more pronounced.
Therefore, for the manufacture of magnetic disks or the like, there has been employed in the prior art a method of randomizing the orientation of the magnetic particles by impressing a magnetic field or the magnetic liquid before the liquid dries so as to prevent the particles from being oriented in some specific direction. Such a method of applying a magnetic field to the magnetic liquid before it dries using an AC magnetic field is disclosed in Japanese Laid-Open Patent Application No. 57-198545.
Moreover, it has been found that the aforementioned process of applying an AC magnetic field is influenced by the liquid composition and the application rate of the liquid, and that the degree of randomization is reduced if the liquid composition and the application rate vary. Even for liquids of supposedly identical compositions, the magnetic conditions for complete randomization vary with delicate variations of the dispersed state of the magnetic power, the magnetic characteristics of the magnetic power, etc., making it very difficult to achieve stable randomization.
The causes for these problems have been investigated by the inventors. As to the first problem, it has been found that there is an optimum frequency for each liquid composition and application rate (i.e., support running speed). That is, it has been discovered that an excessively high frequency restricts the amount of motion of the magnetic particles in the magnetic materials, making it difficult to follow the variations of the magnetic field, and resulting in an orientation of the particles in a perpendicular direction (i.e., transverse direction with respect to the longitudinal direction in which the magnetic field is applied) to the direction of application of the magnetic field. On the other hand, an excessively low frequency causes the particles to be oriented in the direction of application of the magnetic field. Furthermore, as to the second problem, it has been found that maximum randomization takes place at a specific matching point between the liquid composition and the intensity of the magnetic field, and that the degree of randomization is susceptible to delicate variations of the liquid properties such as the dispersion state or the magnetic characteristic of the magnetic power.